WASHINGTON (AP) _ The risk of a rare brain disease and other infections remains a concern for regulators as they seek independent advice on whether to allow a promising multiple sclerosis drug back on the market, according to documents released Monday.

If Tysabri (pronounced ty-SAH-bree) returns to the market, its use should be restricted and patients monitored during treatment and for at least five years thereafter to curtail the risk they run of developing progressive multifocal leukoencephalopathy, or PML, and other opportunistic infections, Food and Drug Administration officials say.

The pharmaceutical companies Biogen Idec Inc. and Elan Corp. PLC voluntarily withdrew Tysabri in February 2005 after three patients developed the rare but often fatal PML. Two of the patients died. About 7,000 patients received the drug during the four months it was on the market, according to the FDA.

Now, the two companies want permission to resume selling the once-a-month IV drug also known as natalizumab, saying its benefits outweigh its risks for patients with relapsing MS. They have proposed a risk-management plan to guard against further cases of the brain disease.

FDA officials said such a program should be beefed up and made mandatory. But they remain skeptical about which patients could safely use the drug.

``Primarily because of the risk of PML, which is not well-quantified, it is unclear for which patients the risk-benefit profile would be acceptable,'' FDA staff wrote in briefing documents released before a meeting of the agency's Peripheral and Central Nervous System Drugs advisory committee.

The outside panel is to spend Tuesday and Wednesday discussing whether to recommend the FDA allow Tysabri back on the market. The FDA typically follows the advice of its advisory committees.

One concern of FDA officials is the ``absence of tools with documented effectiveness to manage'' the risks associated with use of the drug.

The tools, which include regular neurological examinations, MRI scans and the testing of patient spinal fluid and blood serum, might not be useful in monitoring for the presence of the JC virus that is believed to cause PML, FDA staff wrote.

``Hopefully, we will have a far-ranging conversation based on our belief that the risk-benefit profile of Tysabri is still very acceptable and warrants the return of the drug,'' Dr. Burt Adelman, executive vice president, development for Biogen Idec, said of the panel meeting in a recent interview. ``However, we do recognize the risk profile has changed. We have some hypotheses about how to minimize the risk of PML but we will not and have not claimed we understand how to eliminate the risk of PML.''

Multiple sclerosis, a disease of the central nervous system, afflicts about 350,000 Americans. There is no cure and the cause is unknown.

Three studies published last week in the New England Journal of Medicine found Tysabri was relatively safe and quite effective in cutting the rate of relapse in MS patients. In relapses or flare-ups, patients experience recurrent attacks on their nervous system, followed by varying degrees of recovery.

The drug may work by interfering with the movement of inflammatory white blood cells from blood vessels into the brain and spinal cord, according to the FDA.

The single infusion of Tysabri that MS patient Janet Dressell, 52, of Klamath Falls, Ore., received in February 2005, left her less fatigued, better able to walk and articulate her thoughts and allowed her to regain the singing voice she'd lost six years earlier, said her daughter, Christy Cooksey-Dressell.

``That alone was a miracle because we love to hear my mom sing,'' said Cooksey-Dressell, 27, who planned to testify on her mother's behalf. Cooksey-Dressell said MS posed a greater risk than Tysabri to her mother.

``By the time she's 53, if this drug doesn't come back, I can guarantee she will be in a wheelchair. And that devastates me,'' she said.